AP3595

Document number: DS36749 Rev. 1 - 2

15 of 23

www.diodes.com

January 2014

© Diodes Incorporated

AP3595

A Product Line of

Diodes Incorporated

The compensation network is shown in Figure 9. It provides a close loop transfer function with the highest zero cross over frequency and sufficient

phase margin. The transfer function of error amplifier is given by:

)

3

3

1

(

)

2

1

2

2

1

(

}

3

)

3

1

(

1

{

)

2

2

1

(

1

3

1

3

1

)

3

1

3

//(

1

)

2

1

2

//(

1

1

C

R

s

C

C

R

C

C

s

s

C

R

R

s

C

R

s

C

R

R

R

R

sC

R

R

sC

R

sC

V

V

Gain

OUT

COMP

AMP













































The pole and zero frequencies of the transfer function are:

2

2

2

1

1

C

R

f

Z











3

)

3

1

(

2

1

2

C

R

R

f

Z













)

2

1

2

1

(

2

2

1

1

C

C

C

C

R

f

P















3

3

2

1

2

C

R

f

P











R3

+

-

C3

C1

R2

C2

R1

FB

Figure 9.Compensation Network

The closed loop gain of the converter can be written as:

AMP

PW M

LC

Gain

Gain

Gain





Figure 10 shows the asymptotic plot of the closed loop converter gain, and the following guidelines will help to design the compensation network.

Using the below guidelines will give a compensation similar to the curve plotted. A stable closed loop has a-20dB/decade slope and a phase

margin greater than 45 degree.

1. Choose a value for R1, usually between 1k

 and 5k.

2. Select the desired zero crossover frequency.

SW

O

f

f





)

10

/

1

~

5

/

1

(

Use the following equation to calculate R2:

1

2

R

f

f

V

V

R

LC

O

IN

OSC









LC

Z

f

f



 75

.

0

1